Direct dental filling materials consisting of a resin binder and a finely divided inorganic filler are well known to those skilled in the arts. Since the disclosure of such materials in Bowen U.S. Pat. No. 3,066,112, variations and improvements thereon have been disclosed by others, e.g. Gander et al. U.S. Pat. No. 3,385,090 and Lee et al. U.S. Pat. No. 3,926,906.
The direct filling materials to which the present invention pertains employ a resin binder system comprising a polyfunctional monomer having at least two acrylic end groups, as exemplified by 2,2-bis[4-(3-methacryloxy-2-hydroxy-propoxy)-phenyl]-propane (BIS-GMA) and the other monomeric materials taught by the above-identified references, the disclosures of which are hereby incorporated by reference. Also preferably included in the resin system are other active monomers referred to by Bowen as reactive diluents, the function of which is to reduce the viscosity of the resin binder system.
For use as dental filling materials, the resin binders are mixed with a large proportion (generally 65% by weight or more) of a finely divided inorganic filler material having a particle size within the range of about 1 to 85 microns. There is also added to the mixture of the resin binder and the inorganic filler, an appropriate quantity of a free radical generating catalyst, such as benzoyl peroxide, and a suitable activator, such as N,N-di-methyl-para-toluidine. The free radicals which are generated by the combination of the catalyst and the activator lead to rapid polymerization of the resin binder system, producing a dental filling material which has a desirable combination of properties, including high stiffness high compressive strength, low shrinkage on hardening and a low coefficient of thermal expansion, as is known by those skilled in the art.
In preparing direct filling materials of this type, as heretofore known, it has been conventional to treat the finely divided inorganic filler material with an ethyleneically unsaturated organosilane coupling or keying agent. The keying or coupling agent improves the bond between the resin binder and the inorganic filler material, thereby rendering the filler hydrophobic and increasing the strength of the cured dental filling material.